1. Field of the Invention
The present invention relates to the field of fluorescent drive ballasts and more particularly to the field of control processes for driving solid state fluorescent drives. The process taught herein relates to a method for mechanizing the control of drive OFF TIME, and drive ON TIME.
2. Description of Related Art
Recently, fluorescent lamps have been used for back lighting of LCD displays, typically in notebooks and other similar consumer applications as well as for military applications including GPS navigational aids. The lamps for such applications are small and are used alone or in combinations of up to four or more lamps depending on the size of the display. Such lamps have a maximum brightness range of 5:1, and their efficiency is slightly more important than for home or office lighting.
In military, industrial and law enforcement applications, LCD displays using fluorescent lamps are found in aircraft cockpits and other high technology applications. Such applications employ one to forty, or more, lamps in combination and represent examples of high-power density applications with 100 watts or more for a single 6xe2x80x3xc3x979xe2x80x3 display. The information displayed on such displays must be visible in direct sunlight and have a dimming range of over 500:1, and they must operate with high efficiency.
Prior art methods for dimming such light arrays typically vary the duty cycle of the AC drive to the lamp, while keeping the drive frequency constant, or they vary the current to the lamp while maintaining a 100% duty cycle.
Varying the brightness by varying the duty cycle limits the dimming control range. The dimming range is the ratio of the modulation frequency to the lamp drive frequency or frame rate where each sequence of drive pulses occurs within a frame of time of predetermined duration. By way of example, for a typical 40 KHz pulse rate drive, each pulse has a duration of 25 us. If the frame rate is 200 Hz, each frame has a duration of 5 ms which is enough time for 200 pulses having a duration of 25 us. Since the lowest number of integer pulses is one per frame, a dimming range of 200:1 is theoretically possible. However, tests have shown that only 50:1 may be achieved in practice, because lamp flicker develops as the number of pulses in a set is reduced to less than four pulses per set. In addition, as the number of pulses in a set increases from a 1 pulse set, to a 2 pulse set, it can be seen that the power to the lamp per frame is doubled. Even if the flicker problem did not exist, the granularity of the adjustment where fewer than four pulses are provided per frame is therefore inadequate at the minimum brightness levels.
Therefore, a need exists for an optimum dimming control for use in a back light display requiring up to a 10,000:1 brightness range for use with small sized fluorescent lamps in daylight readable displays.
A first advantage of the present invention is that it allows a wide range of control of the lamp""s brightness, with no discontinuities or steps.
A fixed frame rate process is used for the high brightness regime and a variable frame rate process is used for the low brightness regime. The variable frame rate process uses a variable OFF TIME and a fixed ON TIME for each frame period.
The eye is less sensitive to flicker at lower lamp frequencies and at lower rightness levels. The variable frame rate process therefore more closely matches the properties of the eye by providing low brightness levels at low modulation frequencies.
The process of FIG. 6 also reduces or eliminates the effect of discrete changes in brightness at the low brightness levels at the lowest end of the dimming range; a problem common to the fixed frame rate control process. Smooth brightness control with fine resolution is obtained with drive frequencies extending from a transition frequency, at which control changes from a fixed frame rate to a variable frame rate, and extends downward to a lowest frame rate limit, established by a variable OFFTIME. Brightness ranges of over 1000:1 have been obtained.
In a first alternative embodiment, the fluorescent ballast control process includes a low brightness process or routine which uses a variable frame rate with a fixed even number of drive pulses in each frame, to control the brightness of the lamp load over a brightness range extending from a lowest brightness level through a lowest brightness range up to a predetermined intermediate brightness level.
A high brightness control process using a fixed frame rate is used to control the brightness of the fluorescent lamp load over a brightness range extending from a the intermediate brightness level through a high brightness range up to a predetermined maximum brightness level.
The process taught transitions the lamp load brightness from the low brightness control process to the high brightness control process in response to an input signal BRIGHT passing through the control range of values. The transitioning process presented provides for a matched slope at the transition point so that the change from the high to low or low to high brightness range under command of the RIGHT signal is seamless, i.e., without a perceptible jump in brightness as the transition is completed.